In the drilling and completion of an oilwell, it is often desirable to use a down-hole explosive tool. Examples of such down-hole explosive tools include a wireline shooting tool, such as that disclosed in U.S. Pat. No. 3,199,596, and perforators for inserting an orifice insert or flow control device in a well pipe, such as that disclosed in U.S. Pat. No. 3,366,179. Typically, these tools are put in place in the wellbore and an explosive charge in the tool is set off by dropping a weight down the wellbore which strikes a firing head on the upper portion of the down-hole tool. The firing head causes a firing pin to detonate a shell, resulting in the detonation or explosion of the explosive in the tool.
The use of these tools involves considerable uncertainty on the part of the tool operator. In the case of the wireline cutter, it was used to cut the wireline which supported an operating tool which has become stuck or could not be retrieved by the wireline itself. Frequently, when the wireline cutter was dropped by sliding down the wireline, sand or other solids in the oilwell have fallen and packed around the wireline above the cutter. When the weight was thereafter dropped for impact with the cutter to detonate the explosive and thus to cut the wireline, the sand or other solids sometimes prevented the weight from impacting the cutter and thus prevented the detonation of the explosive and the functioning of the cutter, so that the wireline was not cut and could not be retrieved.
One method which was used to prevent the cutter from becoming buried and failing to be set off when the weight was dropped was to drop the weight before the cutter reached the bottom of the hole. However, the sand or other solid material may still have interfered with the weight contacting the cutter or it may have retarded the descent of the tool. Since the weight was smaller than the tool and, thus, its descent was not as retarded by the sand and scale, it dropped faster than the tool. Thus, if the weight was dropped too closely behind the cutter or other down-hole tool, the weight caught up to the tool before it reached the operating tool in the wireline and set it off prematurely so that the wireline was cut at a higher point in the well than desired. When an excess of the wireline above the operating tool was left in the well, difficulties were encountered in the subsequent fishing job in the well to retrieve the stuck operating tool.
One attempt to solve the above problems was the use of a chemical actuated timer device disclosed in U.S. Pat. No. 3,010,515. As disclosed in that patent, a springloaded plunger was used to strike the firing pin. The plunger was prevented from striking the firing pin by a locking pin. Before the tool was dropped, a chemical was released which reacted with the material of the locking pin. When the locking pin become incompetent by the action of the chemical, the plunger struck the firing pin and the explosive tool was activated. Not only was the use of the chemical timer of the patent hazardous to the operator, but reuse required cleaning of the tool, replacement of the locking pin, and placing of a new chemical container in the timer with the potentially dangerous chemical. Further, the reliablity and accuracy of the timing period was dependent on variables, such as, for example, the strength of the chemical, down-hole temperature, and the type and size of the locking pin, which has made the use of the chemical timer of limited practical value. In use, the locking pin might not be completely reacted with the chemical so that the movement of the plunger was retarded, resulting in the firing pin not being struck with enough force to detonate the explosive in the tool.
In contrast, the ignition system of the present invention provides a precise timing mechanism for activating a down-hole tool such as a wireline cutter so that premature firing is prevented, and failure to fire by reason of solids accumulating at the area above the tool is avoided. Further, the ignition system of the present invention may be conveniently and safely prepared for reuse.